Flyback Power Supply Powers Subscriber Line Interface Circuit

  
Inside:
Repository
Cable and xDSL modems are experiencing increasing popularity. For this reason, many designs are now required to interface with existing telephones at the subscriber`s location. The subscriber line interface circuit (SLIC) within the modem has the additional burden of ringing the phone as well as providing loop current while a conversation is takin
Flyback Power Supply Powers Subscriber Line Interface Circuit - schematic

g place. While the phone is ringing and "on-hook, " it appears as an 8k resistance in series with another 1k of capacitive reactance. Typically, the SLIC must be capable of driving this impedance with a 45-VRMS, 20-Hz sinewave with a negative dc-offset in order to ring the phone. Therefore, a high voltage of between ’50 and ’105 V is usually mandated by the SLIC. Once the handset is lifted, the phone places a much lower impedance across the phone terminals, and the SLIC goes into a 20-mA constant-current mode. As a result, the SLIC only needs a power supply of ’24 V. Figure 1 shows the schematic of the power supply for the SLIC portion of the modem. This circuit provides multiple outputs from a single power switch and control IC. It also uses an efficient n-channel MOSFET with low voltage stress. A low input voltage powers a flyback topology. The input source could be either a widely differentiating (generally 2. 5 to 1) output from an ac adapter, or a regulated supply used by some other portion of the system. U2 is the brain of the power supply, as it modulates Q2`s duty factor to control the output voltage. Also, it produces a reference voltage inverting amplifier U1 uses to generate an error signal. The control IC can perform either in current-mode or voltage-mode control. A key advantage of the multi-winding approach is that a single control circuit and single MOSFET can supply two telephony voltages. Good...



Leave Comment

characters left:

New Circuits

.